Growth of Listeria monocytogenes in Skim, Whole and Chocolate Milk, and in Whipping Cream during Incubation at 4, 8, 13, 21 and 35°C.

Autoclaved samples of skim, whole, and chocolate milk and of whipping cream were inoculated with Listeria monocytogenes (one to four strains were tested individually, depending on the experiment) and incubated at 4, 8, 13, 21 or 35°C. Growth curves were then derived and generation times and maximum populations calculated for each combination of strain, product, and temperature. The growth rate of L. monocytogenes was similar in all four products at a given incubation temperature and increased with an increase in temperature. Doubling times over all products and strains were 41 min (35°C), 1 h 43 min-1 h 55 min (21°C), 4 h 27 min-6 h 55 min (13°C), 8 h 40 min-14 h 33 min (8°C), and 29 h 44 min-45 h 33 min (4°C). In each instance, maximum populations reached were at least 10 cells/ml, with highest numbers consistently produced in chocolate milk (at least 10 times greater than in skim or whole milk or cream at any temperature). Little decrease in final numbers occurred with extended storage at the incubation temperature being studied. All results were analyzed statistically to determine magnitude and source of variation. Observed differences in data resulted from interactive effects between strain, product, and temperature. Therefore, no single factor can be considered as the sole cause of a particular finding. That L. monocytogenes can attain such high populations at low temperatures should be of concern. Since refrigerated storage is no guarantee of protection against growth of L. monocytogenes , every precaution should be taken to prevent contamination of certain foods by this organism.